1. Technical Field
The present invention generally relates to arbitration of shared resources within a data processing machine and to dynamically alter priority of those devices assigned to a shifting sequential scheme.
2. Description of the Related Art
Data processing systems typically involve a compilation of various components that interrelate. Typically, in modem data processing systems, one or more devices within the system access a shared resource such as a system bus. This shared resource is one which can only transmit data or communicate with one of the competing devices or requestors at any given time. Therefore, some scheme must be employed to grant authority to use the shared resource when multiple simultaneous requests occur. An arbiter manages selection of which device may use the shared resource when a request or requests are made. The arbiter will select the device to access the shared resource according to some priority configuration, whereby one device takes precedence over another.
In modem computing systems it is a typical design requirement not to allow one particular device to dominate a shared resource. Therefore, two prior art schemes have been instituted in order satisfy this design requirement. These schemes are typically designated as priority and fairness schemes.
It is known that a priority scheme statically assigns priority for each device that has access to a shared resource. When two or more simultaneous requests occur, the arbiter grants access to the device having the highest priority. Once the highest priority device terminates its request, the arbiter grants access to the next highest priority device. The priority of each device under this scheme never changes. Accordingly, a priority scheme creates situations whereby lower priority devices are unable to access the shared resource for extended periods of time.
In a fairness scheme known as Round Robin Arbitration, all resources have the same priority level. However, one device has priority over another device when a device has what is called a "token". In such a scheme, all devices gain priority over all the others in a specified sequence in that the "token" is passed from one device to another in a set order. No device within the Round Robin scheme may access the shared resource without first having this "token". When a device having the "token" is at the top of the order and the requesting device is at the bottom of the order, the "token" must be passed to each intermediate device before reaching the requesting device.
As can be seen by the aforementioned description, a major disadvantage with the Round Robin scheme is that the shared resource remains idle while the "token" is passed to a device requesting access to the shared resource. In relative terms, this lag in time can be long or short depending upon the relative positions of the requesting device and the device with the "token". The Round Robin scheme, therefore, causes inefficient use of the shared resource.
Furthermore, under a Round Robin scheme no weight is given to a device having more critical functions than less critical devices. As a result, critical operations of the data processing system may be delayed because less critical functions tie up the shared resource merely because the less critical device has the "token". Therefore, critical devices have no priority over the other devices in the Round Robin scheme, and as such, the data processing system functions less effectively and efficiently.
Therefore, it would be advantageous to develop a scheme which allows access to requesting devices of lower priority within one clock cycle of the data processing system, while also providing for weighting of priority in favor of more critical devices.